1. Field of the Invention
The present invention relates to an optical disk apparatus for recording information on and/or reproducing the same from an optical disk, a magneto-optical disk or the like where information is optically recordable and reproducible. And more particularly it relates to a focus servo device employed in such an optical disk apparatus.
2. Description of the Prior Art
FIG. 1 is a block diagram of an exemplary constitution for executing a control operation to lock in a focus servo device in a conventional magneto-optical disk apparatus. A first comparator 21 compares a focus error signal, which is based on a detection output from an optical pickup, with a predetermined first reference level ref and generates a high-level comparison output if the result of such comparison signifies that the focus error signal is greater. A second comparator 22 compares the focus error signal with a zero signal level and generates a high-level comparison output if the result of the comparison signifies that the focus error signal is greater. Meanwhile a third comparator 23 compares an RF signal with a predetermined second reference signal level ref2 and generates a high-level comparison output if the RF signal is greater.
An output terminal of the first comparator 21 is connected to a clock terminal CK of a first latch circuit 24, and a high-level state of the signal being applied to an input terminal D thereof is latched every time the output of the first comparator 21 is turned to a high level. Then such high-level state of the signal is delivered from an output terminal Q of the first latch circuit 24. An output terminal of the second comparator 22 is connected to a clock terminal CK of a second latch circuit 25 and, every time the output of the second comparator 22 is turned to a high level, a high-level or low-level state of the signal being applied from the output terminal Q of the first latch circuit 24 to the input terminal D of the second latch circuit 25 is latched and then is delivered from its output terminal Q. Reset terminals CLR of the first and second latch circuits 24, 25 are connected to an output terminal of the third comparator 23. When the output of the third comparator 23 is turned to a low level, the first and second latch circuits 24, 25 are cleared so that outputs thereof are turned to a low level.
In the magneto-optical disk apparatus of the constitution mentioned, the focus servo lock-in control is performed in the following procedure. After a magneto-optical disk (not shown) is loaded in a recording and/or playback unit in the magneto-optical disk apparatus, a light beam is irradiated from an optical pickup (not shown) to the magneto-optical disk. Then in the disk apparatus, an objective lens incorporated in the optical pickup is displaced from a far position to a near position with respect to the magneto-optical disk, and a focus error signal is generated in accordance with the output signal of a photo detector obtained by an astigmatic method through a cylindrical lens which is disposed in the optical path of the reflected light beam from the magneto-optical disk to the photo detector. The relationship between the focus error signal and the distance from the objective lens to the magneto-optical disk is represented by an S curve as shown graphically in FIG. 2. (The astigmatic method is disclosed in, for example, U.S. Pat. No. 4,023,033.)
Subsequently in the first comparator 21, the focus error signal thus generated is compared with a predetermined first signal level ref1 which is lower than the zero signal level (below the zero level in FIG. 2) and, when the result of such comparison signifies that the focus error signal is greater, the output of the first comparator 21 is turned to a high level. Therefore the clock terminal CK of the first latch circuit 24 is turned to a high-level state, and the input terminal of the first latch circuit 24 at that moment is also turned to a high-level state, whereby the output terminal of the first latch circuit 24 is turned to a high-level state. In response to such inversion of the output terminal of the first latch circuit 24 to a high-level state, the objective lens is displaced at a low speed under control of a focus actuator (not shown) incorporated in the optical pickup. More specifically, a low-speed displacement of the objective lens is executed in the vicinity of a point where the focus error signal becomes coincident with a zero level (i.e., in the vicinity of a point P.sub.1 on the S curve of FIG. 2).
In the second comparator 22, its output terminal is turned to a high-level state when the result of the comparison signifies that the focus error signal is coincident with the zero signal level. Consequently the clock terminal of the second latch circuit 25 is turned to a high-level state. Since the output terminal of the first latch circuit 24 is connected to the input terminal of the second latch circuit 25, the input terminal of the second latch circuit 25 is turned to a high-level state at that moment, whereby the output thereof is turned also to a high level. In response to such inversion of the output of the second latch circuit 25 to a high-level state, a loop switch of a focus servo circuit (not shown) is turned on so that an operation for focus servo lock-in control is performed.
The above operation is performed in a state where the output terminal of the third comparator 23, and consequently the reset terminals of both the first and second latch circuits 24, 25, have been turned to a high-level state after the RF signal compared with a predetermined second signal level ref2 in the third comparator 23 is judged to be greater. Namely, the focus servo lock-in control action is executed when the reflected light beam (corresponding to the RF signal) of the initial beam irradiated from the optical pickup to the magneto-optical disk has a sufficient intensity greater than a predetermined value (corresponding to the signal level ref2).
Thus, in the conventional focus servo device, its operation for focus servo lock-in control is performed if the results of the comparisons signify that the focus error signal is coincident with the predetermined zero signal level and that the RF signal is greater than the predetermined signal level. Therefore, in recording information on and/or reproducing the same from a magneto-optical disk which has a recording layer of a small reflectivity, the RF signal level fails to reach the predetermined signal level and raises a problem that the proper operation for focus servo lock-in control is not achievable.